A modern cross country bicycle, or mountain bike, destined to be ridden in much-varying conditions, usually comprises: a pair of knobby tires mounted on thick-spoke rims having a large diameter; a pair of damped front forks, a front triangle and a rear frame, the latter two constituting the bicycle frame, where the rear frame is hinged to the front triangle with the interposing of a rear shock absorber.
The front triangle is constituted by three tubes which form the triangle: a horizontal top tube, a vertical seat tube and an oblique down tube. The saddle post is located in the intersection between the top tube and the seat tube, the saddle post usually being a hollow cylindrical support in which the saddle support is inserted.
The steering tube is located at the other end of the top tube, where it intersects the down tube, the steering tube being a cylindrical cavity which enables housing a steering stem associated to the fork and relative bearings.
A handlebar is associated to the steering stem, the handlebar enabling control of the steering of the front wheel.
A hollow cylindrical seating is present in the intersection between the seat tube and the down tube for housing the bottom bracket, being a device that comprises the spindle to which the crank set is fixed, i.e. the assembly of the cranks and the cogged front chainwheel.
The other component of the frame is the rear frame, which is constituted by a first fork to the stays of which, at the open end of the fork, the free end of the stays of a second fork is fixed; the intersection thereof forms an acute angle and the intersection also affords the holes that function as a seating for a rotation spindle of the rear wheel.
The opposite end to the free end of the first fork is usually hinged to the front triangle in the bottom bracket zone, while the closed end of the second fork is hinged to the front triangle at a point usually below the saddle and above the intersection point between the saddle tube and the down tube. The stays of the first fork are known as lower chain stays, while the stays of the second fork are termed higher chain stays.
In this way the frame can be conceived as a union of a front triangle which forms the main frame and a second triangle which forms the rear frame of the suspension, hinged to one another and between which a shock absorbing element is interposed, though other geometries are conceivable.
On varying the ride conditions, which range from steep descents to sharp ascents, the distribution of the load between the front wheel and the rear wheel changes radically. When riding on flat terrain about 70% of the weight bears on the rear wheel and only the remaining 30% bears on the front wheel. Differently, when riding uphill, almost 100% of the weight force unloads on the rear wheel, leaving the front wheel with a very light load, and capable of lifting from the ground even with only minimum stresses. On the contrary, when riding downhill, it can happen that almost 100% of the weight force is concentrated on the front wheel, a situation that can lead to a raising, even an involuntary one, of the rear wheel.
In recent decades, the evolution and popularity of off-road cycling practices in courses featuring varied terrain have led manufacturers to develop specialized bicycles conceived for facilitating downhill or uphill rides, appropriately suiting the frame geometries to these specific ride situations.
In particular, bicycles designed for facilitating the uphill ride have geometries designed so as to optimize the balancing of the weight force in order to facilitate partial displacement of the rear wheel towards the front wheel, by adopting a more vertical steering tube and a saddle post that is more vertical with respect to the bottom bracket. These bicycles, developed with the objective of making uphill pedaling more efficient, assume the characteristic of having a smaller distance between the bottom bracket and the spindle of the front wheel, the containing of which facilitates the displacement of the weight on the front wheel, and makes the bicycle easier to handle.
Vice versa, bicycles developed with the aim of maximizing downhill performance include opposite bicycle geometries, i.e. they facilitate, thanks to a less vertically-inclined steering angle and a saddle post further back with respect to the vertical of the bottom bracket, a substantially retracting of the center of gravity towards the rear wheel, facilitating the downhill rideability of the bicycle and limiting the tendency of the rear wheel to detach from the ground in the steepest descents. Structurally these bicycles are also characterized by a distance between the bottom bracket and the spindle of the front wheel that is greater than that of uphill bicycles, making them more stable on rough terrain, but less easy to handle.
A further feature distinguishing the two different types of above-described bicycles is the height of the bottom bracket. Downhill bicycles in fact have bottom brackets positioned lower with the aim of lowering the center of gravity of the assembly constituted by the bicycle with a cyclist on board.
Positioning the bottom bracket as low as possible is always useful from the point of view of the handling and stability of the bicycle, but there is a limit given by the need to be able to pedal even in the presence of obstacles on the course without the pedals touching the obstacles; in downhill bicycles the need to pedal is limited, so it is possible to position the bottom bracket lower with respect to a normal mountain bike.
If an analysis is made with reference to a downhill bicycle, the characteristics lead to having a bicycle that enables the cyclist to feel at ease in the descent; on the other hand the mountain bike realized in this way is hard to exploit when on flat terrain or when moving uphill. For example the combination of the bicycle geometries, highly specialized, with the use of suspensions having a long travel, means having a bicycle that bounces during pedaling, so that much of the energy from the pedaling action is lost and, given an equal force applied, progress is slower.
For these reasons, a downhill rider is often led to use mechanical means, for example trucks and/or cable cars, in order to reach a point in which to begin the descent; this, among other things, limits the use of these bicycles to places that can be reached by road or cable car.
Prior art solutions exist that enable varying some of the geometric heights of the frame or the rear suspension of the bicycles, in order to make the bicycle adaptable to both descent and ascent, by adapting the frame to the course that the rider is to ride.
With reference to the U.S. Pat. No. 7,712,757, a Mountain Bike is known to be constituted by a stiff element which constitutes the main frame and by a rear frame connected thereto by means of a hinge system provided with a shock absorbing element. The shock absorbing element (shock-absorber) is connected via a rotating pin with the rear frame by a first rocker link. The second (and lower) anchoring point of the shock-absorber is pivoted to the down tube of the main frame by means of an intermediate element. The intermediate element, or second rocker link, functions as a housing for the bearing of the main joint, on which the rear frame is pivoted. Therefore the main rocker link of the rear frame (lower chain stay) is not connected directly to the main frame, but is connected thereto indirectly via a connecting element. Further, the described structure in the above-cited patent is provided with an elastic element arranged between the connecting element (second rocker link) and the saddle tube, which by means of a contraction/extension thereof enables displacing the connecting element and with it the lower anchoring point of the main shock absorbing element (shock-absorber). Configured in this way the solution enables the anchoring point to displace autonomously forward and backward under the effect of the tension of the chain exerted at the time of pedaling (forwards when force is exerted on the pedals and backwards during the step of compression and/or braking without the action of the pedals. The constraints of dependence on the damping behavior of the rear suspension with respect to the tension of the chain are however often negatively perceptible by the cyclists, and as such make these systems open to discussion. This is true in particular during off-road riding, when the rear frame, subjected to frequent and extreme stresses by the roughness of the terrain, frequently displaces. Further, the overall rigidity of the frame is reduced due to the presence of an additional damping element during movement.
Further, the solution described in this patent requires significant displacements of the lower anchoring point of the shock absorbing element, in the order of 40-50 mm, in order to enable the system to have a significant effect in terms of variation of the geometric heights and the functioning of the suspension, large displacements that lead to a solid design of the mobile elements at the cost of a reduced rigidity of the whole, and also lead to a destabilization of the bicycle during pedaling, making the ride less precise as these variations are not predictable and not controllable by the cyclist.
There exists a further solution described in U.S. Pat. No. 6,877,591, in which in order to change the position of the second anchoring point of the shock absorbing element to the down tube of the frame a second fixing element is used that can be fixed to the down tube in different positions. This fixing position can be varied only by demounting the fixing pin of the mobile element to the frame with suitable tools and remounting it in a different position. By doing this the anchoring point of the shock absorbing element can be varied, but this operation is made uncomfortable by the need to use tools and in that this cannot be done during the ride.
Further, the arrangement of a shock absorbing element between a front triangle and a rear frame is known from patent GB 2 360 497. In this solution the anchoring of the shock-absorber to the rear frame does not occur by means of a rocker link connection but is realized directly. The second anchoring of the shock absorbing element is positioned directly on the down tube of the front triangle. In order to change the geometry of the frame, the first anchoring point of the shock-absorber on the rear frame can be displaced internally of an appropriately profiled ride guide. As with the preceding reference the limitation of this solution is the necessity of using tools for carrying out the operation, as well as the fact that it cannot be done while the vehicle is being ridden.
A further solution is known from patent EP No. 2603418, which includes a mechanism able to vary the position of the upper attachment of the shock absorbing element with respect to the rocker link, of the rear suspension. In this solution the shock absorber is indirectly connected to the rear frame via a rocker link pivoted on the saddle tube of the front triangle. The variation of the upper attachment point of the shock absorber on the rocker link, which is made possible by means of a remotely-actuatable hydraulic system activated by a command on the handlebar, enables varying some geometric heights of the frame on which it is installed. However, this variation varies the overall position of the front triangle with respect to the rear frame, inducing a variation of equal measure on both the angle of inclination of the steering tube and on the angle of inclination of the saddle post (the front triangle being fixed). Further this variation has no significant impact on the wheel base (the distance between the spindle of the rear wheel and the spindle of the front wheel), enabling a partial adaptation of the bicycle geometry to the changing ride situations.
The problem of this solution therefore lies in the fact that the variation in the configuration obtainable by acting only on the attachment position of the rear suspension is limited, and therefore, though there are benefits, the difficulty of using a bicycle for uphill riding that was conceived for downhill use is only minimally mitigated.
Other prior art example are described in documents US 2011/227312, WO 2015/051472, US 2013/093160 and U.S. Pat. No. 5,628,524, but in all these prior art solutions there is no provision for the bottom bracket to be displaced with respect to the main frame and, further, the allowed variation of the bicycle geometry is always free and unblocked when the bicycle is running.
An aim of the present invention is to obviate the above-mentioned drawbacks of the prior art, with a solution that is simple and rational.
The aims are attained by the characteristics of the invention as reported in the independent claim. The dependent claims delineate preferred and/or particularly advantageous aspects of the invention.